Bisphenol A polycarbonate is a well known and widely used engineering thermoplastic having utility for producing molded articles and protective plastic overcoats. It is known to exhibit good hardness, good impact resistance, and, in its amorphous higher molecular weight forms, good transparency. However, bisphenol A polycarbonate has drawbacks for some applications, namely, poor resistance to stress cracking caused by contact with solvents such as gasoline or other chemicals, and, in its lower molecular weight forms, a tendency to crystallize and thereby become hazy rather than transparent.
In contrast, some thermoplastic polyesters, while not having as high impact resistance as bisphenol A polycarbonate, have excellent resistance to stress cracking caused by contact with solvents or other chemicals, and good amorphousness and transparency over a wider range of molecular weights.
It would be desirable to be able to provide a polymeric composition, for example by blending a polyester with bisphenol A polycarbonate, that would provide a combination of the beneficial properties of both the polyester and bisphenol A polycarbonate, e.g., good hardness, high impact resistance, high transparency, good resistance to chemical-caused stress cracking, and relatively reasonable cost. Unfortunately, these goals have been thwarted in the past by the inherent incompatibilities of many polyesters with bisphenol A polycarbonate. By "incompatibility", we means the inability of the two polymers to form and/or maintain a solid solution with each other over a wide range of proportions and external conditions. Incompatibility of a blend of two or more polymers is evidenced by the blend's having more than one Tg, which is a reliable indicator that the polymers in the blend exist in separate phases, each exhibiting properties different from each other, rather than the single set of properties exhibited by a blend of compatible polymers that form a solid solution with each other, i.e., form a single phase in the blend. Inherent drawbacks of phase separations between the polymers in a blend include: inability to provide a single Tg, poorer structural integrity, poorer resistance to impact and other stress, poorer resistance to cracking caused by chemical attack, and poorer transparency due to light scattering, all of which are contrary to the purposes intended in creating the blend.
Thus, a need exists for a blend of a polyester and bisphenol A polycarbonate, wherein the blended polymers are compatible with each other over a wide range of proportions, and wherein the blend exhibits good amorphousness and transparency, good impact resistance, a single Tg, better resistance to chemically caused stress cracking than that of the bisphenol A polycarbonate itself, and reasonable cost.